Public telephone systems have generally used the same system architecture for over one hundred years. Conventional systems employ highly centralized and hierarchical switching facilities as nodes that tie together a complex web of connected links. While a very small portion of their subscribers use direct links to geostationary satellites, the vast majority of the links that couple the conventional phone network comprise a bewildering and dissimilar assortment of cables, wires, fibers and microwave repeaters. Like all hierarchical systems, conventional telephone networks are extremely vulnerable when the performance of any high-ranking node in the hierarchy is impaired. For example, a fire in a Hinsdale, Ill. wire center denied long-distance service to the Chicago area for several days. Relatively minor software failures can disrupt or even suspend service in large, densely-populated urban areas. This vulnerability to localized failure is compounded by the limits which a centralized architecture imposes on the expansion potential of the network. On average, every time a new subscriber is added to a conventional land-based communication network, expensive additions to switching hardware and connecting wires must be installed. The marginal cost of adding each new subscriber is extremely high, and the cost of raising that capital is a drain on telephone subscribers and on the economy at large. Conventional telephone systems are inherently circumscribed by their hierarchical design, and these limits now impose critical barriers to the enormous augmentation of capacity which the previous networks must provide to meet the burgeoning world demand for communications services in the coming decades.
Several attempts to bypass these inherent limits have met with mixed results. Large consumers of phone services have begun to install their own private networks to carry large volumes of voice messages, video, and broadband data calls. While some of these expensive and awkward enhancements provide partial solutions, the unalterable constraints imposed by a centralized switching topology continue to confine the future growth of existing networks.
Some extension of the century-old centralized telephone switching infrastructure has been achieved using geostationary satellites. These spacecraft, however, offer additional communications capabilities that are quite limited. Since these satellites operate in equatorial orbits, they are not accessible to customers located in high latitudes. Because they must share their orbit with many other services, their number is restricted to a relatively small population. Since all of these spacecraft occupy a single circular orbit, they can not be connected together in a geodesic network. A geodesic network, which could provide enormously greater capacity, must be generally spherical in shape. Geostationary satellites also suffer from a very serious disadvantage--the distant altitude of their orbits. These satellites are so far from Earth that the signal takes about one-quarter of a second to traverse the nearly 50,000 miles (80,000 km) along the round trip from the ground up to the satellite, and back to the ground. The delays sensed by the telephone user's ear that are introduced by this long round trip are not only annoying, but can render some conversations which are relayed between more than one geostationary satellite virtually unintelligible. Radio signals which are exchanged between a ground station and a geosynchronous satellite may also be impaired by this great round trip distance. A telephone customer on the ground using a portable phone who wanted to communicate directly with a satellite in geostationary orbit would need a telephone capable of producing an output in excess of hundreds of Watts. Generating this power output is not only thoroughly impractical for users of portable phones, but may also create a radiation hazard for the individual wielding the telephone.
Geostationary satellites do not supply an adequate solution to the formidable expansion needs of conventional telephone networks. Although cellular service has grown rapidly over the past decade, even greatly expanded cellular service would not provide an adequate solution. Cellular systems are plagued by poor performance, and are ultimately constrained by the same structural limits that circumscribe the future of land-based systems. Cellular customers are still limited to geographic regions served by radio towers called "cell sites." Even in the United States, these cell sites are not universally prevalent, since market forces restrict cellular service to only the most densely populated urban portions of our country. Cellular service is available to only a small minority of privileged users in wealthy countries, and is virtually non-existent in lesser developed parts of the world.
The publications noted below disclose various systems that pertain to communication systems that are designed to operate on the Earth's surface or in conjunction with satellites flying in low Earth orbits.
Bertiger, Leopold and Peterson describe a "Satellite Cellular Telephone and Data Communication System" in European Patent Application No. 891 184 58.2. This application sets out some of the details of Motorola's proposed Iridium.TM. communication system. The Iridium.TM. system is currently designed to utilize sixty-six (66) satellites in low Earth orbit which would generate relatively large footprints of radio beams due to their extremely low mask angle of eight and one half degrees (81/2.degree.). Because of these very large footprints, the communications capacity that may be offered by the Motorola network would be substantially constrained. In addition, this system would employ "satellite-fixed cells" which are not defined by any constant boundaries on the Earth. These cells would sweep over vast regions of the Earth at very high speeds as the Iridium.TM. satellites fly overhead. This method of using satellite-fixed cells introduces extremely complicated "hand-off" problems when one satellite moves out of range of supplying service with a subscriber. At that time, another satellite must assume the responsibility of supporting the subscriber's call without interruption.
In U.S. Pat. No. 5,107,925, Bertiger et al. disclose a multiple beam space antenna system for facilitating communications between a satellite switch and a plurality of Earth-based stations.
No system that is currently available to the general public is capable of taking advantage of the enormous enrichment of communications capacity that could be achieved if the traditional centralized grid of terrestrial switches, and their connecting cables, wires, fibers and microwave repeaters could be completely bypassed. Public phone companies are not presently able to sell continuous global service to customers who wish to use phones that are not coupled to the land-based network. The problem of providing an economically feasible network for voice, data and video which can be used by subscribers all over the world has presented a major challenge to the communications business. The development of a communications system that offers a solution to the immutable obstacles to growth which bind conventional phone networks would constitute a major technological advance and would satisfy a long felt need within the telephone industry.